Dr. Kasahara proposes a novel vector system that consists of a hybrid of adenovirus and retroelements, such as retroviruses and retrotransposons. The adenovirus-retrovirus hybrid system utilizes a helper-dependent adenoviral vector as a carrier to deliver the retroviral or retrotransposon vector. As a completely "gutted" helper- dependent adenoviral delivery system is used, there will be no inadvertent expression of adenoviral structural genes. The secondary expression of the retroelement vector will continue transiently as long as the adenoviral vector persists in the transduced cells, leading to stable long-term integration of the transgene into the host cell genome in the case of a second-stage retrotransposon, and into the genomes of adjacent dividing cells in the case of a second-stage retrovirus, thus greatly increasing the overall efficiency of transduction.